1. Field of the Invention
The present invention generally relates to a driving apparatus for light emitting diodes (LEDs), and more particularly, to a driving apparatus for LEDs without employing an electrolytic capacitor.
2. Description of Related Art
Since the last 20 years, people have been keeping developing new types of illumination light sources. The European Union specifically worked out “Rainbow Project”, wherein it is addressed that a new type light source must meet the following four conditions: high-efficiency, energy-saving, pollution-free and resembling natural light. In fact, LEDs have the above-mentioned advantages so that any traditional illumination light sources (for example, incandescent bulb and fluorescent lamp) are uncompetitive with the LEDs. In this regard, LEDs are commonly recognized as the “green” light source with the most value in the 21st century, and the LEDs would substitute the incandescent bulbs and fluorescent lamps to dominate the illumination product market.
In the present time, LEDs are mainly applicable to mega-size display, universal illumination, laser device, LCD backlight source, illumination for instrument and meter, and pattern identification and so on. Along with the rapid progress of high brightness LEDs, a more critical requirement must be fulfilled on the driving technique of LEDs, wherein in order to fully take advantage of the merit of the semiconductor illumination, the AC-DC driver for the LEDs needs to include the following advantageous: high efficiency, low cost, high power factor and long lifetime.
In terms of traditional LED driving, various schemes are available, such as current-limiting by using resistor, linear adjustment, charge pump converting control and switch converter control. With a daily illumination circumstance having a commercial AC (alternating current) voltage input, the AC-DC driver architecture for high power LEDs can be roughly shown by, for example, FIG. 1. According to the Energy-Star standard, the input power factor (IPF) of an AC-DC driver for commercial luminaries must be no less than 0.9, while the IPF of an AC-DC driver for residential luminaries, no less than 0.7. Accordingly, the commercial AC voltage Vac must be processed by a bridge-type rectifier 101 and a power factor correction converter 103 (PFC converter 103) so as to implement the PFC and the AC-DC converting to provide the DC-DC converter of the successive stage with a 24V or 12V stable voltage. In this way, the LED driving chip 107 is able to provide a constant current for stable operation of the large power LEDs 109.
Although the architecture of the AC-DC driver shown by FIG. 1 can ensure the large power LEDs 109 to have better light-emitting quality, but the above-mentioned design architecture comes with many disadvantages: too many components, larger volume and short lifetime. For example, assuming the IPF of the PFC converter 103 is 1, both the input current Iin and the input voltage Vin herein are sin-waves with the same phase as shown by FIG. 2A. Since the input power Pin at the time takes sin-square waveform, to realize a constant-voltage and constant-current output (i.e., constant output power Po, as shown by FIG. 2B), an electrolytic capacitor C with a large capacitance is required so as to realize the balance between the input power Pin and the output power Po. It should be noted that the electrolytic capacitor C usually has a lifetime of 5,000 H (hour), which does not match the much longer lifetime of 50,000 H of the LEDs. It is obvious that the electrolytic capacitor C becomes the major factor to shorten the total lifetime of the LED AC-DC driver.